Serotonin inputs to inspiratory laryngeal motoneurons in the rat

Nocturnal stridor in multiple system atrophy: Video-polysomnography and clinical features

INTRODUCTION

Nocturnal stridor, a life-threatening condition linked to respiratory failure and sudden death during sleep, is a serious issue in patients with multiple system atrophy (MSA). However, little is known about polysomnographic findings and clinical features of MSA patients with nocturnal stridor. Hence, we investigated video-polysomnography (VPSG) findings and clinical features associated with nocturnal stridor in patients with MSA.

METHODS

We retrospectively analyzed the clinical data of patients with MSA (n = 49) who underwent overnight VPSG for the evaluation of sleep-disordered breathing. The presence of nocturnal stridor was confirmed based on overnight VPSG findings. Clinical data, including VPSG findings and clinical features, were compared between MSA patients with and without nocturnal stridor.

RESULTS

Nocturnal stridor was present in 31 (63.3%) patients with MSA. Patients with stridor showed significantly higher apnea-hypopnea, respiratory disturbance, and oxygen desaturation indices than those without stridor (P = 0.024, P = 0.049, and P = 0.006, respectively). Patients with stridor had more severe axial motor features, more impaired activities of daily living, and longer disease duration than those without stridor (P = 0.012, P = 0.036, and P = 0.003, respectively). However, there were no significant between-group differences in sex, age at disease onset, MSA subtype, parkinsonian features, cerebellar ataxia, residual urine volume, or systolic and diastolic blood pressure change.

CONCLUSIONS

MSA with nocturnal stridor is related to higher apnea indices in conjunction with higher O₂ desaturation index, more severe axial motor features, more impaired activities of daily living, and longer disease duration.

Serotonin and substance P: Synergy or competition in the control of breathing

Numerous neurotransmitters identified in the central nervous system play role in ventilatory control. This mini-review focuses on the respiratory effects of two neurotransmitters: serotonin (5-HT) and substance P (SP). We discuss their co-localization in medullary raphe nuclei, expression of proper receptors within the specific regions of respiratory related structures and contribution to respiratory rhythmogenesis.

Isolated Voice Tremor: A Clinical Variant of Essential Tremor or a Distinct Clinical Phenotype?

Background

The consensus statement by the Task Force on Tremor of the International Parkinson and Movement Disorder Society excludes individuals with "isolated voice tremor" as a clinical variant of essential tremor (ET). This clinical viewpoint presents a rationale for reconsideration of "isolated voice tremor" as a clinical variant of ET.

Methods

Evidence from the literature was extracted to characterize the clinical phenotype of "isolated voice tremor," or essential vocal tremor (EVT). Clinical features were extracted from relevant literature available at pubmed.gov using the terms "EVT," "essential voice tremor," "primary voice tremor," and "organic voice tremor."

Results

The average age of onset in those with EVT was older than 60 years (range 19-84 years), with 75-93% being female. The typical duration of vocal tremor ranged from 1 to 13 years (average 6 years). The distribution of structures exhibiting tremor included the larynx, soft palate, pharynx, and base of tongue in the majority of patients, with some exhibiting tremor of the head and respiratory musculature. The condition of tremor occurred during speech and quiet respiration in 74% of individuals. Rate of tremor ranged from 4 to 10 Hz. Nearly 70% reported onset of vocal tremor prior to upper limb involvement. Family history of tremor was reported in 38-42% of individuals.

Discussion

Those previously classified with EVT demonstrate a similar familial history, rate, tremor classification, and body distribution of ET. EVT is proposed as a clinical variant of ET in the pattern of onset and progression of body distribution from the midline cranial to spinal neural pathways.

Vocal cord electromyographic correlates of stridor in multiple system atrophy phenotypes

INTRODUCTION

Multiple system atrophy (MSA) is a neurodegenerative disorder characterized by dysautonomia in combination with parkinsonian and cerebellar signs. Stridor may also occur and it is associated with life-threatening events and poor prognosis. The pathophysiology of stridor in MSA is still debated.

OBJECTIVE

To define correlations between diurnal electromyographic (EMG) abnormalities of vocal cord muscles and stridor in MSA phenotypes.

METHODS

We recruited 60 patients with "probable" MSA (45 with parkinsonian [MSA-P] and 15 with cerebellar phenotype [MSA-C]). Nocturnal stridor was detected with video-polysomnography, whereas diurnal stridor was clinically noted when present. A diurnal kinesiologic EMG study of the adductor thyroarytenoid and the abductor posterior cricoarytenoid muscles was also performed.

RESULTS

Among subjects with nocturnal stridor, MSA-P patients predominantly showed a paradoxical burst-like activation of the adductor thyroarytenoid muscle during inspiration. This dystonic pattern was associated with nocturnal stridor in MSA-P (odds ratio [OR] = 23.64, 95% confidence interval [CI] 3.42-70.77, p < 0.001). Conversely, MSA-C patients with nocturnal stridor mainly had additional neurogenic findings of vocal cord muscles. This dystonic-plus pattern correlated with nocturnal stridor in MSA-C (OR = 17.21, 95% CI 4.17-74.92, p < 0.01). The findings of diurnal stridor paralleled the observations for nocturnal stridor.

CONCLUSIONS

The pathophysiology of stridor may differ between MSA phenotypes, possibly related to dysfunctional supranuclear mechanisms in MSA-P (dystonic pattern) and to additional nuclear damage in MSA-C (dystonic-plus pattern).

Do selective serotonin reuptake inhibitors improve survival in multiple system atrophy?

INTRODUCTION

Loss of brainstem serotonergic neurons in MSA patients is implicated in respiratory dysfunction including stridor and may increase the risk of sudden death. Augmenting serotonergic transmission through selective serotonergic reuptake inhibitors (SSRIs) has been proposed to improve stridor and prolong survival in multiple system atrophy (MSA). We sought to determine whether MSA patients on an SSRI during their disease course have improved survival compared to those not on an SSRI.

METHODS

Review of all MSA patients from 1998 to 2012 at Mayo Clinic, Rochester who completed autonomic function testing. Use of SSRI medications was obtained from patient-provided medication lists in the electronic medical record. Clinical symptoms were collected from patient histories; the presence of stridor was obtained from clinical histories and polysomnogram. Surviving patients were called to assess for stridor and SSRI use.

RESULTS

Of 685 MSA patients, 132 (19%) were on an SSRI. Median time from symptom onset to death was 7.5 years with no difference based on SSRI use (p = .957). Rates of stridor were similar in SSRI users and non-users based on patient report and polysomnography (p = .494 and p = .181, respectively). SSRI use was associated with parkinsonism (p = .027) and falls (p = .002). Stridor was similar in SSRI users and those not on an SSRI.

CONCLUSIONS

There was no difference in survival in MSA patients on an SSRI. However, SSRI use was associated with higher rates of parkinsonism and falls.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Intermittent hypoxia-induced cardiorespiratory long-term facilitation: A new role for microglia

Intermittent hypoxia induces plasticity in neural networks controlling breathing and cardiovascular function. Studies demonstrate that mechanisms causing cardiorespiratory plasticity rely on intracellular signalling pathways that are activated by specific neurotransmitters. Peptides such as serotonin, PACAP and orexin are well-known for their physiological significance in regulating the cardiorespiratory system. Their receptor counterparts are present in cardiorespiratory centres of the brainstem medulla and spinal cord. Microglial cells are also important players in inducing plasticity. The phenotype and function of microglial cells can change based on the physiological state of the central nervous system. Here, we propose that in the autonomic nuclei of the ventral brainstem the relationship between neurotransmitters and neurokines, neurons and microglia determines the overall neural function of the central cardiorespiratory system.

Medullary mediation of the laryngeal adductor reflex: A possible role in sudden infant death syndrome

The laryngeal adductor reflex (LAR) is a laryngeal protective reflex. Vagal afferent polymodal sensory fibres that have cell bodies in the nodose ganglion, originate in the sub-glottal area of the larynx and upper trachea. These polymodal sensory fibres respond to mechanical or chemical stimuli. The central axons of these sensory vagal neurons terminate in the dorsolateral subnuclei of the tractus solitarius in the medulla oblongata. The LAR is a critical, reflex in the pathways that play a protective role in the process of ventilation, and the sychronisation of ventilation with other activities that are undertaken by the oropharyngeal systems including: eating, speaking and singing. Failure of the LAR to operate properly at any time after birth can lead to SIDS, pneumonia or death. Despite the critical nature of this reflex, very little is known about the central pathways and neurotransmitters involved in the management of the LAR and any disorders associated with its failure to act properly. Here, we review current knowledge concerning the medullary nuclei and neurochemicals involved in the LAR and propose a potential neural pathway that may facilitate future SIDS research.

Recurrent laryngeal nerve activity exhibits a 5-HT-mediated long-term facilitation and enhanced response to hypoxia following acute intermittent hypoxia in rat

A progressive and sustained increase in inspiratory-related motor output (“long-term facilitation”) and an augmented ventilatory response to hypoxia occur following acute intermittent hypoxia (AIH). To date, acute plasticity in respiratory motor outputs active in the postinspiratory and expiratory phases has not been studied. The recurrent laryngeal nerve (RLN) innervates laryngeal abductor muscles that widen the glottic aperture during inspiration. Other efferent fibers in the RLN innervate adductor muscles that partially narrow the glottic aperture during postinspiration. The aim of this study was to investigate whether or not AIH elicits a serotonin-mediated long-term facilitation of laryngeal abductor muscles, and if recruitment of adductor muscle activity occurs following AIH. Urethane anesthetized, paralyzed, unilaterally vagotomized, and artificially ventilated adult male Sprague-Dawley rats were subjected to 10 exposures of hypoxia (10% O2 in N2, 45 s, separated by 5 min, n = 7). At 60 min post-AIH, phrenic nerve activity and inspiratory RLN activity were elevated (39 ± 11 and 23 ± 6% above baseline, respectively). These responses were abolished by pretreatment with the serotonin-receptor antagonist, methysergide ( n = 4). No increase occurred in time control animals ( n = 7). Animals that did not exhibit postinspiratory RLN activity at baseline did not show recruitment of this activity post-AIH ( n = 6). A repeat hypoxia 60 min after AIH produced a significantly greater peak response in both phrenic and RLN activity, accompanied by a prolonged recovery time that was also prevented by pretreatment with methysergide. We conclude that AIH induces neural plasticity in laryngeal motoneurons, via serotonin-mediated mechanisms similar to that observed in phrenic motoneurons: the so-called “Q-pathway”. We also provide evidence that the augmented responsiveness to repeat hypoxia following AIH also involves a serotonergic mechanism.

Cholinergic inputs to laryngeal motoneurons functionally identified in vivo in rat: a combined electrophysiological and microscopic study

The intrinsic laryngeal muscles are differentially modulated during respiration as well as other states and behaviors such as hypocapnia and sleep. Previous anatomical and pharmacological studies indicate a role for acetylcholine at the level of the nucleus ambiguus in the modulation of laryngeal motoneuron (LMN) activity. The present study investigated the anatomical nature of cholinergic input to inspiratory- (ILM) and expiratory-modulated (ELM) laryngeal motoneurons in the loose formation of the nucleus ambiguus. Using combined in vivo intracellular recording, dye filling, and immunohistochemistry, we demonstrate that LMNs identified in Sprague-Dawley rat receive several close appositions from vesicular acetylcholine transporter-immunoreactive (VAChT-ir) boutons. ELMs receive a significantly greater number of close appositions (mean ± standard deviation [SD]: 47 ± 11; n = 5) than ILMs (32 ± 9; n = 8; t-test P < 0.05). For both LMN types, more close appositions were observed on the cell soma and proximal dendrites compared to distal dendrites (two-way analysis of variance [ANOVA], P < 0.0001). Using fluorescence confocal microscopy, almost 90% of VAChT-ir close appositions (n = 45 boutons on n = 4 ELMs) were colocalized with the synaptic marker synaptophysin. These results support a strong influence of cholinergic input on LMNs and may have implications in the differential modulation of laryngeal muscle activity.

Substance P, tyrosine hydroxylase and serotonin terminals in the rat caudal nucleus ambiguus

Substance P (SP), tyrosine hydroxylase (TH) and serotonin inputs onto laryngeal motoneurons (LMNs) are known to exist, but the distribution of their terminals in the caudal nucleus ambiguus (NA), remains unclear. Using immunofluorescence and confocal microscopy, we assessed simultaneously the distribution of SP, TH, serotonin and synaptophysin immunoreactive (ir) terminals in the caudal NA. SP, TH and serotonin-ir varicosities were considered to represent immunoreactive synapses if, using confocal microscopy, they were co-localized with the presynaptic protein, synaptophysin. Relative to the total number of synapses, we found only a modest number of SP, TH or serotonin-ir synaptic terminals in the caudal NA. The density of SP-ir synaptic terminals was higher than that of TH-ir and serotonin-ir synaptic terminals. Our results suggest that SP, TH, and serotonin-ir inputs may play only a modest role in regulating the activity of LMN. We conclude that SP, TH and serotonin are not always co-localized in terminals forming inputs with LMN and that they arise from separate subpopulations of neurons.

The role of serotonin in respiratory function and dysfunction

Serotonin (5-HT) is a neuromodulator-transmitter influencing global brain function. Past and present findings illustrate a prominent role for 5-HT in the modulation of ponto-medullary autonomic circuits. 5-HT is also involved in the control of neurotrophic processes during pre- and postnatal development of neural circuits. The functional implications of 5-HT are particularly illustrated in the alterations to the serotonergic system, as seen in a wide range of neurological disorders. This article reviews the role of 5-HT in the development and control of respiratory networks in the ponto-medullary brainstem. The review further examines the role of 5-HT in breathing disorders occurring at different stages of life, in particular, the neonatal neurodevelopmental diseases such as Rett, sudden infant death and Prader-Willi syndromes, adult diseases such as sleep apnoea and mental illness linked to neurodegeneration.

The close relationship between life-threatening breathing disorders and urine storage dysfunction in multiple system atrophy

Survival of multiple system atrophy (MSA) depends on whether a variety of sleep-related breathing problems as well as autonomic failure (AF) occur. Since the brainstem lesions that cause respiratory and autonomic dysfunction overlap with each other, these critical manifestations might get worse in parallel. If so, the detection of AF, which is comparatively easy, might be predictive of a latent life-threatening breathing disorder. In 15 patients with MSA, we performed autonomic function tests composed of postural challenges and administered a questionnaire on bladder condition, as well as polysomnography and laryngoscopy during wakefulness and under anesthesia. Polysomnographic variables such as the apnea-hypopnea index (AHI) and oxygen saturation (SpO(2)) and the findings of laryngoscopy were compared with the degree of cardiac and urinary autonomic dysfunction. AHI, mean SpO(2) and the lowest SpO(2) showed significant correlations with urine storage dysfunction. In addition, patients with vocal cord abductor paralysis (VCAP) or central sleep apnea (CSA) contributing to nocturnal sudden death had more severe storage disorders than those without. On the other hand, no significant relationship between polysomnographic variables and orthostatic hypotension was observed except in the case of mean SpO(2). These results indicate that life-threatening breathing disorders have a close relationship with AF, and especially urine storage dysfunction. Therefore, longitudinal assessment of deterioration of the storage function might be useful for predicting the latent progress of VCAP and CSA.

Differential regulation of the central neural cardiorespiratory system by metabotropic neurotransmitters

Central neurons in the brainstem and spinal cord are essential for the maintenance of sympathetic tone, the integration of responses to the activation of reflexes and central commands, and the generation of an appropriate respiratory motor output. Here, we will discuss work that aims to understand the role that metabotropic neurotransmitter systems play in central cardiorespiratory mechanisms. It is well known that blockade of glutamatergic, gamma-aminobutyric acidergic and glycinergic pathways causes major or even complete disruption of cardiorespiratory systems, whereas antagonism of other neurotransmitter systems barely affects circulation or ventilation. Despite the lack of an 'all-or-none' role for metabotropic neurotransmitters, they are nevertheless significant in modulating the effects of central command and peripheral adaptive reflexes. Finally, we propose that a likely explanation for the plethora of neurotransmitters and their receptors on cardiorespiratory neurons is to enable differential regulation of outputs in response to reflex inputs, while at the same time maintaining a tonic level of sympathetic activity that supports those organs that significantly autoregulate their blood supply, such as the heart, brain, retina and kidney. Such an explanation of the data now available enables the generation of many new testable hypotheses.

Depletion of medullary serotonergic neurons in patients with multiple system atrophy who succumbed to sudden death

Multiple system atrophy (MSA) is a neurodegenerative disorder characterized by prominent autonomic failure with ataxia and/or parkinsonism. The leading cause of death in MSA is sudden death. We have shown that the early development of autonomic failure is an independent risk factor for sudden death. The depletion of sympathetic preganglionic neurons in the spinal intermediolateral cell column (IML) and its afferent medullary catecholaminergic and serotonergic neurons has been proposed to be partly responsible for autonomic failure in MSA. In this study, we investigated whether the depletion of neurons in any of these autonomic neuron groups contributes to sudden death in MSA. Out of 52 autopsy-proven patients with MSA, we selected 12 individuals who had died within 3.5 years after disease onset to define the accurate levels of slices and identify early neuropathological changes of autonomic nuclei in MSA. Four patients succumbed to sudden death and eight patients died through established causes. Serial 10 mum sections were obtained from the 8th segment of the thoracic cord and the rostral medulla oblongata. Sections from the medulla oblongata were immunostained for thyrosine hydroxylase and tryptophan hydroxylase. The total cell number in the five sections was computed for comparison. Compared with the control, the MSA group showed a marked depletion of neurons in the IML (38.0 +/- 7.1 versus 75.2 +/- 7.6 cells, P < 0.001), thyrosine hydroxylase-immunoreactive neurons in the ventrolateral medulla (VLM) (17.4 +/- 5.1 versus 72.8 +/- 13.6 cells, P < 0.01) and tryptophan hydroxylase-immunoreactive neurons in the VLM (15.6 +/- 9.2 versus 60.8 +/- 17.0 cells, P < 0.01), nucleus raphe obscurus (19.3 +/- 4.4 versus 75.3 +/- 8.6 cells, P < 0.001), nucleus raphe pallidus (2.1 +/- 2.7 versus 9.0 +/- 3.4 cells, P < 0.03), and arcuate nucleus (0.4 +/- 0.8 versus 2.3 +/- 1.5 cells, P < 0.05). Moreover, in patients who succumbed to sudden death, when compared with patients who had established causes of death, we found a marked depletion of tryptophan hydroxylase-immunoreactive neurons in the VLM (7.3 +/- 3.5 versus 21.8 +/- 6.5 cells, P < 0.02) and nucleus raphe obscurus (15.0 +/- 2.0 versus 22.5 +/- 2.1 cells, P < 0.01). The results indicate that the spinal IML and medullary catecholaminergic and serotonergic systems are involved even in the early stages of MSA, and the dysfunction of the medullary serotonergic system regulating cardiovascular and respiratory systems could be responsible for sudden death in patients with MSA.

Serotonergic projections from the caudal raphe nuclei to the hypoglossal nucleus in male and female rats

The respiratory control system is sexually dimorphic. In many brain regions, including respiratory motor nuclei, serotonin (5HT) levels are higher in females than in males. We hypothesized that there could be sex differences in 5HT input to the hypoglossal nucleus, a region of the brainstem involved in upper airway control. Adult Fischer 344 rats were anesthetized and a retrograde transsynaptic neuroanatomical tracer, Bartha pseudorabies virus (PRV), was injected into the tongue. Sections through the medulla were reacted immunocytochemically for the presence of (i) PRV, (ii) tryptophan hydroxylase (TPH; marker of 5HT neurons), (iii) PRV combined with TPH, and (iv) 5HT. Sex hormone levels were measured in female rats and correlated with TPH immunoreactivity, as hypoglossal 5HT levels vary with the estrous cycle. The number of PRV neurons was comparable in male and female rats. The number and distribution of TPH immunoreactive neurons in the caudal raphe nuclei were similar in male and female rats. The subset of 5HT neurons that innervate hypoglossal motoneurons was also similar in male and female rats. With the exception of the ventrolateral region of the hypoglossal nucleus, 5HT immunoreactivity was similar in male and female rats. These data suggest that sex differences in 5HT modulation of hypoglossal motoneurons in male and female rats are not the result of sex differences in TPH or 5HT, but may result from differences in neurotransmitter release and reuptake, location of 5HT synaptic terminals on hypoglossal motoneurons, pre- and postsynaptic 5HT receptor expression, or the distribution of sex hormone receptors on hypoglossal or caudal raphe neurons.

Arrest of 5HT neuron differentiation delays respiratory maturation and impairs neonatal homeostatic responses to environmental challenges

Serotonin (5HT) is a powerful modulator of respiratory circuitry in vitro but its role in the development of breathing behavior in vivo is poorly understood. Here we show, using 5HT neuron-deficient Pet-1 (Pet-1(-/-)) neonates, that serotonergic function is required for the normal timing of postnatal respiratory maturation. Plethysmographic recordings reveal that Pet-1(-/-) mice are born with a depressed breathing frequency and a higher incidence of spontaneous and prolonged respiratory pauses relative to wild type littermates. The wild type breathing pattern stabilizes by postnatal day 4.5, while breathing remains depressed, highly irregular and interrupted more frequently by respiratory pauses in Pet-1(-/-) mice. Analysis of in vitro hypoglossal nerve discharge indicates that instabilities in the central respiratory rhythm generator contribute to the abnormal Pet-1(-/-) breathing behavior. In addition, the breathing pattern in Pet-1(-/-) neonates is susceptible to environmental conditions, and can be further destabilized by brief exposure to hypoxia. By postnatal day 9.5, however, breathing frequency in Pet-1(-/-) animals is only slightly depressed compared to wild type, and prolonged respiratory pauses are rare, indicating that the abnormalities seen earlier in the Pet-1(-/-) mice are transient. Our findings provide unexpected insight into the development of breathing behavior by demonstrating that defects in 5HT neuron development can extend and exacerbate the period of breathing instability that occurs immediately after birth during which respiratory homeostasis is vulnerable to environmental challenges.

Brainstem respiratory control: substrates of respiratory failure of multiple system atrophy

Multiple system atrophy may manifest with severe respiratory disorders, including sleep apnea and laryngeal stridor, which reflect a failure of automatic control of respiration. This function depends on a pontomedullary network of interconnected neurons located in the parabrachial/Kölliker Fuse nucleus in the pons, nucleus of the solitary tract, and ventrolateral medulla. Neurons in the preBötzinger complex expressing neurokinin-1 receptors are critically involved in respiratory rhythmogenesis, whereas serotonergic neurons in the medullary raphe and glutamatergic neurons located close to the ventral medullary surface are involved in central chemosensitivity to hypercapnia, hypoxia, or both. Pathological studies using selective neurochemical markers indicate that these neuronal groups are affected in multiple system atrophy. This finding may provide potential anatomical substrates for the respiratory manifestations of the disease.

Simultaneous inhibition of caudal medullary raphe and retrotrapezoid nucleus decreases breathing and the CO2 response in conscious rats

The medullary raphe (MR) and the retrotrapezoid nucleus (RTN) in the ventral medulla are two of many central chemoreceptor sites. We examine their combined function in conscious rats by focal inhibition using microdialysis. Inhibition of RTN neurons with the GABA(A) receptor agonist muscimol, with simultaneous dialysis of artificial cerebrospinal fluid (ACSF) in or near to the caudal MR, causes hypoventilation (decrease in the ratio of minute ventilation to oxygen consumption, V(E)/V(O2)) and reduces the ventilatory response to 7% CO(2) by 24%. Inhibition of caudal MR serotonergic neurons with the 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), with simultaneous dialysis of ACSF in or near to the RTN, causes hypoventilation but has no significant effect on the CO(2) response. Inhibition of both the RTN and the caudal MR simultaneously produces enhanced hypoventilation and a 51% decrease in the CO(2) response. The effects of treatment on the CO(2) response are similar in wakefulness and in non-rapid eye movement sleep. Comparison of the effect of 8-OH-DPAT microdialysed into a more rostral portion of the MR, where the CO(2) response is reduced by 22%, demonstrates heterogeneity within the MR of the function of serotonergic neurons in breathing. We conclude that serotonergic neurons within the caudal MR provide a non-CO(2)-dependent tonic drive to breathe and potentiate the effects of RTN neurons that contribute to a resting chemical 'drive to breathe' as well as the response to added CO(2). These effects of caudal MR serotonergic neurons could be at a chemoreceptor site, e.g. the RTN, or at 'downstream' sites involved in rhythm and pattern generation.

Effects of electrical stimulation of the medullary raphe nuclei on respiratory movement in rats

The present study was undertaken to examine the effects of electrical stimulation of the medullary raphe nuclei on respiration in rats anesthetized with ketamine and xylazine. Train pulse stimuli (100 Hz, 10-30 microA) were applied in the regions of the caudal raphe nuclei: the raphe magnus (RM), raphe pallidus (RP) and raphe obscurus (RO). Stimulation of the RM depressed inspiratory movements measured by means of an abdominal pneumograph, whereas stimulation of the RP augmented inspiratory movements. It was revealed that stimulation of the RO induced either inhibitory or facilitatory effects on respiratory movements depending on the stimulation sites. These findings confirm and extend previous studies concerning the effects of raphe stimulation on respiratory activity in cats. The present results demonstrate that in rats the caudal raphe nuclei are involved in respiratory control.

Serotonin inputs to laryngeal constrictor motoneurons in the rat

OBJECTIVES/HYPOTHESIS

The objective was to demonstrate close appositions between serotonin-immunoreactive boutons and laryngeal constrictor (LCon) motoneurons in Sprague-Dawley rats.

STUDY DESIGN

Animal experimental.

METHODS

LCon motoneurons were identified functionally by their antidromic responses to stimulation of the recurrent laryngeal nerve and postinspiratory modulation and were filled by intracellular injection of biotin amide (n = 6). The medulla was sectioned and, using immunohistochemical analysis, examined by light microscopy.

RESULTS

Serotonin appositions were found on all 6 LCon motoneurons, with an average number of 17 +/- 6 close appositions per neuron.

CONCLUSION

In comparison with the authors' previous study of inspiratory laryngeal motoneurons, the number of serotonin close appositions with LCon motoneurons was similar to that found with posterior cricoarytenoid motoneurons, but significantly less than that found with cricothyroid motoneurons. This finding may represent a basis for differences in tonic activity of laryngeal muscles observed in relation to the sleep-wake cycle.

Membrane potential changes in vocal cord tensor motoneurons during breathing, vocalization, coughing and swallowing in decerebrate cats

We studied the patterns of membrane potential changes in vocal cord tensor motoneurons, i.e. cricothyroid muscle motoneurons (CTMs), during fictive breathing, vocalization, coughing, and swallowing in decerebrate paralyzed cats to determine the nature of central drives to CTMs during these behaviors. CTMs were identified by antidromic activation from the superior laryngeal nerve. During breathing, CTMs always depolarized during the inspiratory phase, and sometimes depolarized during the expiratory phase as well. During vocalization, CTMs strongly depolarized. During coughing, CTMs exhibited depolarizations during both inspiratory and expiratory phases, but it was interrupted by a transient repolarization between the last part of the inspiratory phase and the first part of the abdominal burst during which chloride-dependent inhibitory postsynaptic potentials were revealed. During swallowing, most CTMs hyperpolarized, and this hyperpolarization was sometimes followed by a weak depolarization. We conclude that the main role of the cricothyroid muscle is vocalization but the functional roles in coughing and swallowing are minor, and that the CTM activity during resting breathing and vocalization are primarily controlled by excitatory inputs, while during coughing and swallowing, inhibitory inputs play roles in shaping membrane potential trajectories.

Substance P inputs to laryngeal motoneurons in the rat

Substance P terminals have previously been demonstrated around retrogradely labelled posterior cricoarytenoid (PCA) motoneurons, but little is known regarding substance P inputs to other functionally identified laryngeal motoneurons. In the present study, we determined the number and distribution of close appositions between substance P immunoreactive boutons and three types of laryngeal motoneuron by using a combination of intracellular recording, dye-filling and immunocytochemistry in the rat. Cricothyroid (CT) motoneurons received 15+/-5 substance P appositions/neuron (mean+/-S.D., n = 6), PCA motoneurons received 13+/-5 (n = 6), and laryngeal constrictor (LCS) motoneurons received 11+/-4 (n = 5). In contrast to our previous finding of a preferential serotonin innervation of CT motoneurons, we found no significant difference between the substance P inputs to CT, PCA and LCS motoneurons. Our results indicate a modest role for substance P in control of laryngeal motoneuronal function.

Activity patterns of cardiac vagal motoneurons in rat nucleus ambiguus

Extracellular recordings were made in the right nucleus ambiguus of urethane-anesthetized rats from 33 neurons that were activated at constant latency from the craniovagal cardiac branch. Their calculated conduction velocities were in the B-fiber range (1.6-13.8 m/s, median 4.2), and most (22/33) were silent. Active units were confirmed as cardiac vagal motoneurons (CVM) by the collision test for antidromic activation and by the presence of cardiac rhythmicity in their resting discharge (9/9). Brief arterial pressure rises of 20-50 mmHg increased the activity in five of five CVM by 0.1 +/- 0.02 spikes. s(-1). mmHg(-1) from a resting 3.8 +/- 1.2 spikes/s; they also recruited activity in two of four previously silent cardiac branch-projecting neurons. CVM firing was modulated by the central respiratory cycle, showing peak activity during inspiration (8/8). Rat CVM thus show firing properties similar to those in other species, but their respiratory pattern is distinct. These findings are discussed in relation to mechanisms of respiratory sinus arrhythmia.